Preparation of silicon tetrafluoride



Jan. 17, 1961 F. oLsTowsKl 2,958,599

PREPARATION OF SILICON TETRAFLUORIDE Filed Sept. 2. 1958 @il N INV ENT OR.

@22PM/V9 HTTORA/Ey United States Patent O 2,968,599 PREPARATION F SILICON TETRAFLUORIDE Franciszek Olstowski, Freeport, Tex., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Sept. 2, 1958, Ser. No. 758,436 9 Claims. (Cl. 20461) This invention relates to a process for preparation of silicon tetrauoride, and more particularly, to the preparation of this compound by the electrolysis of a nonvolatile molten metal uoride containing a metal silicate or silica.

This application is a continuation-in-part of patent application Serial No. 663,966, filed on .lune 6, 1957, now abandoned.

Metal iluorides, metal silicates and silica are cheap raw materials and a process whereby silica tetrafluoride can be prepared by the electrolysis of these fused metal salts considerably reduces the production cost of this compound.

It is, therefore, among the objects of this invention to provide a process for the preparation of silicon tetrauoride by electrolysis of a fused metal fluoride containing a metal silicate or silica.

The above and other objects are attained by passing an electric current through a molten electrolyte between a porous carbon anode andan insoluble cathode where the electrolyte consists essentially of a mixture of a metal fluoride which is non-volatile and stable at the electrolysis temperature selected from the group consisting of alkali metal uorides, `alkaline earth metal fluorides, earth metal fluorides, and mixtures thereof and a metal silicate which is non-volatile and stable at the electrolysis temperature or silicon dioxide.

It has been discovered that when a porous carbon an- 0de is used in the electrolysis of an electrolyte consisting essentially of the particular metal fluoride or fluorides and a metal silicate or silica, an anode product is obtained containing silicon tetrafluoride. The anode product will contain a number of fluorine-containing compounds including both saturated and unsaturated fluorocarbons as well as the silicon tetraliuoride. The anode product may be gaseous at the electrolysis temperature, but may contain higher molecular weight uorocarbons which are oils at room temperature. With a porous carbon anode, the fluorine-containing compounds are obtained by the electrolysis without encountering anode affect.

The porous carbon anode used may be an intimately combined solid mass type which is made by combining amorphous carbon, such as petroleum coke, coal, carbon black etc. or allotropic carbon, such as graphite, with a binder and sintering the mixture to form an intimately combined solid mass having a prescribed permeability. Also, the porous anode may consist of carbonaceous material in particulate form conned so that the individual particles are in electrical contact with each other. A porous carbon anode which is intimately combined by sintering to form a solid porous cohered mass is generally preferred.

A solid mass type porous anode having a permeability of `at least one and not greater than 40 is generally used. It is preferred that the permeability be in the range of `4 to 20. While an anode having a permeability less than one may he used in special cases, no beneficial advantage is obtained. The maximum anode current density which may be used without encountering anode effect is proportional to the permeability, increasing with an increase in permeability. With the permeabilities generally used, in the range of l to 40, normally all practical anode current densities may be used without encountering this objectionable phenomenon. In special cases, however, where relatively low current densities are to be employed an anode having a permeability as low as 0.2 may be used, if desired. An anode having a permeability greater than 40 is seldom used. The structural strength of the anode decreases with the porosity which makes it less desirable than a less porous anode.

Permeability as used herein, refers to the porous anodes which are intimately combined in a solid mass by sintering and is expressed as the amount of air passing through the porous carbon anode in cubic feet per minute per square foot per inch thickness at a pressure differential of 2 inches of water. The term porous, as used herein, means gas permeable.

While the current eiciency and the yield of silicon tetratluoride may not be as great, a porous anode comprising of carbonaceous material in particulate form loosely confined is less costly and thus may be desirable in some cases. Practically any carbonaceous material in particulate form may be used. Charcoal, coke, lamp black,p0wdered carbon, and powdered graphite are illustrative examples of some of the carbonaceous materials which are operative. Due to its availability petroleum coke in particulate form is preferred. Generally particles of the carbonaceous material larger than one inch are not used except in a large unit where a large bed is employed. Particles as small asthose passing through a No. 200 standard mesh screen and being retained on a No. 300 mesh screen are operative. However, it is preferred to use carbonaceous material which will pass through a No. 6'standard mesh screen and be retained on a No. 40.

The invention may be more easily understood when the detailed discussion is considered in conjunction with the drawings, in which:

Figure l diagrammatically illustrates an electrolytic cell employing a porous anode comprising of carbonaoeous material in particulate form loosely conned which Vmay be used in carrying out the invention, and

Figures 2 and 3 show different types of porous anode assemblies which may be used in the cell shown in Figure 1 when a solid mass type porous anode is used.

The electrolytic cell diagrammatically shown in Figure 1 comprises a metal tank 1 having a cover plate 2 and an electrical non-conducting cylindrical liner 3 in which the electrolyte 4 is placed, and a carbon lead 5 extending part way into the tank through anopening in cover plate 2. As shown, cover plate 2 is fastened to tank 1 by means of a multiplicity of screws 6 to form a gas tight seal. Clamps or other means may be used. A pipe 7 is inserted in an opening 8 in cover plate 2 and provides a passageway through which the anode gas produced as a product inside of tank 1 may be withdrawn from the tank. The attachment of pipe 7 to cover plate 2 is gas tight and may be obtained by welding the outer periphery of the pipe to the cover plate or by having the end which is inserted in the cover plate threaded and the opening 8 also threaded to receive the pipe. Where the carbon lead 5 passes through cover plate 2, an electrical insulating seal 9' is used so that the gas tight seal is obtained. An electrical lead 10 through which the current is supplied to the cell is attached to the carbon lead at the end which is not inserted into the tank. Another lead 11 is electrically connected to the surface ofthe tank which through a mol- `separate the particular iiuorocarbons obtained.

`assembly shown in Figure 2.

.tencathode12 at the bottom of the tank completes the be deposited is lighter than the electrolyte. This simplies the construction of the cell, since no provisions have .tobe made to entrap or remove the metal deposited from the surface of the electrolyte. Carhonaceous material 13 1n particulate form which being lighter than the molten .electrolyte oats on the surface surrounding and in contact vwith carbon lead 5. Carbon lead 5 is not immersed in the electrolyte itself.

In the operation of the cell, the cell is heated to the desired temperature. When the desired temperature is `obtained an electrical potential is applied to leads and 11 to provide a current ow through-the electrolyte. The anode product whichl may be substantially all gaseous is withdrawn from within the tank through pipe 7. lt is then further processed by knownV methods to recover and The metal depositing out from the electrolyte is deposited in the molten cathode'and later recovered by known methods.

Figures 2 and 3 illustrate a porous anode assembly wherein a solid mass type of a porous anode is used.Y As shown in Figure 2 the anode assembly comprises a cylin- .drical carbon or graphite anode holder having a passageway 21 extending through the center along its longitudinal axis. A hollow-cup shaped piece of porous carbon 22 is attached to the lower end of holder 20 with passageway 21 communicating with a hollow inside area :23 ofthe porous cup 22. At the upper end of holder 20, a pipe 24 is inserted in passageway 21 and thus provides a means by which the anode product forming at the porous cup .may be removed from the cell. A lead 25 is attached `to holder 21 and thus supplies the current to the anode and provides a means for completing the circuit in the cel vFigure 3 shows a modification of the porous anode It comprises a carbon or graphite carbon holder which at its lower end 31 is `enlarged. The holder has a passageway 32 extending `along its longitudinal axis which becomes enlarged Vat the lower end. The porous anode 33 in the shape of a plug is inserted in the lower end of the anode holder 30 in the enlarged area of the passageway. A pipe 34 is inserted in passageway 32 at the upper end of the anode holder. In the operation of the cell the porous anode portion of the anode assembly as shown in Figures 2 and 3 are immersed in the electrolyte. Y

The shape of the porous carbon anode used is immaterial. The hollow-cup type as shown in Figure 2 or the plug type as shown in Figure 3 are preferred especially where higher molecular weight fluorocarbons may be obtained. These uorocarbons may be readily drawn through the porous anode and removed from the system through the passageway in the holder. Instead of using ahollow-cup type porous anode as shown in Figure 2 or a plug as shown in Figure 3, a solid cylindrical piece of the porous carbon material may be used as an anode. Itmay be vnecessary in some cases to use a hood or shield to enclose the solid anode to entrap the anode gases as they are formed and released in order to remove them from the system.v Other types of anode assemblies which are apparent to those skilled in the art may also be used.

' VIllustrative examples of the alkali metal, alkaline Vearth metal and earth'metal fluorides which may be used-as the fluoride constituent of the'electrolyte are magnesium uoride, aluminum fluoride, sodium iluoride, barium-fluoride, strontium fluoride, calcium uoride, lithium fluoride, and cesium fluoride. These metal vfluorides are non-volatile and stable at electrolysis temperature. Generally in .the electrolysis the same ilumine-containing compounds are obtained regardless of the particular metal' fluoridel used as the 'uoride constituent o f the electrolyte. AHowever, the ratio of the silicon tetrauoride obtained in the anode product may vary somewhat with the metal fuoride employed. Although only one of the alkali metal fluorides, alkaline earth metal uorides, or earth metal uorides may be used as the uoride constituent of the electrolyte, a mixture of these metal fluorides is often used to increase the conductivity or lower the melting point of the bath. For this purpose, lithium ii'uoride is most commonly added to the other metal iluorides, but other mixtures and combinat'ions may also be used. When other uorides are added to either increase conductivity or lower vthe melting point of a particular metal fluoride bath, the `iluorides of metalswhich arehigher in the electromotive series or more electronegative Vthan the metal to be extracted at the cathode from the particular bath are preferred. By using uorides of metals more electronegative, these metals will not deposlt out at the cathode with the desired metal except at exceptionally high cathode current densities. Thus, the cathode productis not contaminated under normal operation conditions. Also in'continuous operation'of the cell, the metal fluoride added to the electrolyte is not depleted by the electrolysis and only the iluorideofV the particular metal being deposited .at the cathode has toqbe added continuously. For example, when lithium fluoride is added to a magnesium uoride bath, the lithium is more electronegative than magnesium and thus will not deposit out at the cathode. Once the lithium uoride is added to the bath it will not be depleted by the electrolysis and only magnesium iluoride has to he added for continuous operation.

Illustrativev examples of the metal uoride mixtures that -may be used and the metal preferentially deposited outat the cathode are shown in the table below.

Metal preferentially deposited at the To the metal iluoride or mixture of uorides, silica or a metal silicate which is non-volatile and stable at the electrolysis `temperature is added. By the electrolysis of the resulting electrolyte consisting essentially of the metal uoride or metal uorides and the silicon containing compound, silicon tetrafluoride is obtained. When a metal silicate is used as a constituent in the electrolyte, it is preferred to add the silicate of the same metal as that of the fluoride constituent of the electrolyte which is being deposited at the cathode in the bath. Thus, the cell can be continuously operated by just adding the metal iluoride electrlyte and the particular silicate. A nonvolatile and stable silicate of the same metals as the alkali metals,valkaline earth metals, or earth metals of the uoride which may bev used other than the particular metal utilized in'the bath may also be used. However, the metal :of the silicate may .continuously increase inu the cell, if it isl higher in the electromotive series, or deposit out at the cathode with the desired metal. of the bath, if lower. In special cases, non-volatile and stable silicates ofmetals other than those otl the uorides which may d be used in the electrolyte may also be used.` Mostof 'posit out at the cathode in conjunction with the electrolyte metal. Illustrative examples of these other metal silicates are manganese metasilicate and lead metasilicate.

The energy required for the production of the fluorine- 'containing compound is generally higher than for the production of compounds containing other anions. Conjsequently, a higher concentration of the fluoride in the bath is generally maintained to obtain a greater formation of the fluorine-containing compounds. Generally, fa mole ratio of the uoride to the silicate or silica in the range of 10:1 to 80:1 is used, preferably in the range of 20:1 to 40:1.

While an anode product containing a higher percentage of silicon tetrauoride is normally obtained at lower temperatures, an electrolysis temperature in the range 'of 700 to 1000 C. is generally used. The optimum of 'the temperature for a particular electrolyte may vary somewhat.' rfl`he minimum temperature that may be employed is the melting point of the electrolyte used, since the electrolyte must be in the molten state. The maximum temperature is either limited by cell structure, the stability and volatility of the particular electrolyte employed or metal silicate added to the bath, or the thermal 4stability of the particular uorine compound desired at the anode. Since at a lower temperature the construction of the cell is simplified, a temperature in the range of 700 to 1000 C. is thus generally preferred within which range an anode product containing a relatively high percentage of silicon tetrauoride is obtained. However for an electrolyte containing a LiF-NaFAlF3 mix- -ture as the fluoride constitute, the optimum temperature `-is somewhat lower being in the range of 650 to 800 C.

Although anode current density below 1 and up to 100 amperes per square inch may be used with certain porous anodes, an anode density in the range of 1 to 40 amperes is generally employed, preferably in the range of 1 to 10 amperes per square inch. Generally, at a higher anode current density, the anode product contains a higher percentage of silicon tetralluoride. With `an anode of carbonaceous material in particulate form loosely confined, an anode current density of over 5 amperes per square inch is seldom used due to the high voltage required. The cathode current density is generally in the range of l to 30 amperes per square inch. To obtain the current densities desired a voltage up to 30 volts may be employed, but a voltage in the range of 4 to l0 is preferred. For the loosely conned anode, a higher voltage may be required to obtain the desired anode current density.

Various electrolytic cell construction and various types of anodes which are apparent to those skilled in the art may be used. The particular anode adopted will generally depend upon the metal being deposited in the cell.

The term earth metals, as used herein, means the elements aluminum, scandium, yttrium and lanthanum of the third group of the periodic system.

The term stable, as used herein in reference to the metal fluoride and metal silicates, means salts which are thermally stable and will not decompose due to temperature itself.

The term non-volatile as used herein in reference to the metal fluoride and metal silicates, means salts which do not have a vapor pressure in excess of 20 mm. of Hg at the electrolysis temperature.

The following example further illustrates the invention but is not to be construed as limiting it thereto.

Example I An electrolytic cell was employed in the preparation of silicon tetratluoride. The cell comprised a steel crucible with an alumina liner and was similar to that shown in Figure 1 except that a porous anode assembly similar to that shown in Figure 3 was used instead of the particulated carbonaceous material. The porous anode plug was approximately ll/z inches in diameter and 1% inches long.

To the cell, approximately 1200 grams of an electrolyte comprising 52 weight percent sodium uoride and 48 weight percent of lithium fluoride were added. The electrolyte was heated to 725 C. and 600 grams of lead added to serve as the cathode. The cell was operated for approximately one hour at current of 20 amperes and 9 volts. Then 16 grams of calcium silicate were added and stirred into the bath. The cell was operated for an additional hour at a current of around 20 amperes and the anode gas was collected during this period.

The analysis of the anode gases indicated the following analysis in mole percent:

In a manner similar to above, Si02, LigSiOa, MgSiOs, and other non-volatile and stable silicates may be substituted for the calcium silicate to obtain similar anode products. Also other fluoride electrolytes such as MgFz-LiF, MgF2NaFCaF2 may be employed in place of the NaF-LiF electrolyte used above. i

Example Il In a cell similar to that described in Example I approximately 1000 grams of a mixture containing 48 weight percent lithium lluoride and 52 weight percent sodium fluoride were added to the cell. Approximately 410 grams of lead were added to the cell to act as a molten cathode. After the cell was heated to a temperature of 650 C., 50 grams of sodium metasilicate (NazSlOa) were added. An anode assembly, similar to that of Example I except that the porous anode plug was l inch in diameter and 1 inch long, was immersed in the electrolyte. A current of 20 amperes at `a potential of 24 volts was passed through the electrolyte for 1% hours while the electrolyte temperature was maintained in the range of 650 to 710 C. The anode product obtained was analyzed by infra-red. The anode product contained the following in mole percent on an air-free basis.

What is claimed is:

1. A process for the preparation of silicon tetrauoride which comprises passing an electric current through an electrolyte between a porous carbon anode and an insoluble cathode at a temperature sufficient to melt the electrolyte to o-btain an anode product containing silicon tetrauoride, said electrolyte consisting essentially of a mixture containing at least one metal uoride which is non-volatile and stable at the electrolysis temperature selected from the group consisting of alkali metal uorides, alkaline earth metal fluorides, and earth metal uorides and a compound selected from the group consisting of metal silicates which are non-volatile and stable at the electrolysis temperature and silica, and recovering the silicon tetrafluoride from the anode product.

2. A process for the preparation of a silicon tetrauoride which comprises passing an electric current through an electrolyte between a porous carbon anode and an insoluble cathode at a temperature sufcient to melt the electrolyte to obtain an anode product containing silicon tetrauoride, said electrolyte consisting essentially of a mixture containing at least one metal fiuoride which is non-volatile and stable at the electrolysis temperature selected from the group consisting of alkali i1n thget g the. isill ,Qn tetratfluo herein l the ,electro- "rdiils te ,elaim .2. S les. 1y lef el least one alleeli metal .fluich `is I nlatile and stable at `Y*electrolysis r and@ :tiptel silicate which ,is `nori-.voletile "ii "s't''b'le at kelectrp ysis temperature mgleelei 2. herein the eleeirelyte consists essentially" of t least yone Valkaline earth metal uoride which is non-volatile and stable at elecv'"troly'sis temperature and a metal silicate which lis nonlylo'lzitile and stable at electrolysis temperature.

RiSf-Aprocess according-to claim 2 wherein theelectro- Ijlyie consists essential-ly of at least one earth metal fluo- Qr'ide which `is non`volatile andstable A:it electrolysis temfraturel and `a metal silicate --which ,lis non-Volatile and stable at electrolysis temperature.

6.- A preeess ,eeeenrling e .Gleim ..2 -Wliereln ille eleeifeselllielly 0f e fnlixtliie 9i en. .alkali metal metal fluoride end a metal k e and V stable at electrolysis temperature,

7L A process ferllle nrenerellerl ef .Silieen tetfafluorlde which comprises passing an electric current through an .eleetrelyte between. a porous .carbon ...anode .lend en. il luvble cgtholeat a temperaturesuicient to melt the t the anode .containing -tetraiiuoride `ssii electrfllyte consisting v.esse tilly ieete which .is noti-.volatileendtable.et the .eleetrQlySli lem- Qgp @ture in .en e 'oimt YStich hat .the mele relie ,Oif `the 'llllcride to the .silicate in the ve'lectrolytefis inpthe reuse Qf 202.110 49:1..

.8- Arproeesslaeeorflingto claim? wherein themetal ,silicate is .calcium silicate, the permis carbon eno/de is an intimately` combined solid mass having a permeability in the Vrangeof 4to 20, and the electrolyte is at altem- Yperjfitureintherange of 700 to l00 0 C. v

$9- Afproeess aceerding te claim 7 wherein themetel silleate iS. .Sodium silicate, the :remue-.carbon anode isan intimately combined. solid meefeihevins alperme 4ili'ty of at leastO-l, .and V the.electrolyte is .e1-e. temperature fin. the

.llelereiieee -Citedin :the-file `of thiefnateiir :UNITED vSTATES PATENTS 

1. A PROCESS FOR THE PREPARATION OF SILICON TETRAFLUORIDE WHICH COMPRISES PASSING AN ELECTRIC CURRENT THROUGH AN ELECTROLYTE BETWEEN A POROUS CARBON ANODE AND AN INSOLUBLE CATHODE AT A TEMPERATURE SUFFICIENT TO MELT THE ELECTROLYTE TO OBTAIN AN ANODE PRODUCT CONTAINING SILICON TETRAFLUORIDE, SAID ELECTROLYTE CONSISTING ESSENTIALLY OF A MIXTURE CONTAINING AT LEAST ONE METAL FLUORIDE WHICH IS NON-VOLATILE AND STABLE AT THE ELECTROLYSIS TEMPERATURE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL FLUORIDES, ALKALINE EARTH METAL FLUORIDES, AND EARTH METAL FLUORIDES AND A COMPOUND SELECTED FROM THE GROUP CONSISTING OF METAL SILICATES WHICH ARE NON-VOLATILE AND STABLE AT THE ELECTROYSIS TEMPERATURE AND SILICA, AND RECOVERING THE SILICON TETRAFLUORIDE FROM THE ANODE PRODUCT. 